Process and a device for measuring static and dynamic friction of sheet-shaped materials

ABSTRACT

A process for measuring the friction and the coefficient of friction of sheet-shaped materials (layer materials), espeically paper. This process is characterized in that a test surface of the material and a conter-surface of a material, in relation to which it is desired to measure the friction, in said testing process are loaded with a normal (transverse) force which is applied against the test surface of the sheet-shaped material, the position of said normal force in the space being maintained essentially stationary in the testing process, and a friction force acting in the direction of the tested surface, and determining the combination of firction force acting in the direction of the test surface and normal force, which can be applied without mutual movement (sliding) or are required respectively, for sliding movement between the test surface and the counter-surface, for determing a resting friction (static friction) and a sliding friction (kinetic friction) between the test surface and the counter-surface.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention is related to a process for measuring the friction anddetermining the coefficient of friction of sheet-shaped materials,especially paper and similar materials, especially based on cellulose,e.g. from wood, such as softwood or hardwood, but also paper from otherkinds of cellulose materials and also paper from other materials as wellas other sheet-shaped materials of organic or inorganic materials, alsoincluding metals or similar materials, as well as plastics, elastomers,fabrics, such as woven or non-woven fabrics, etc.

The expressions "sheet", "sheet-shaped" and "layer" herein is on thefirst hand intended to comprise a body having an extension in onedirection which is small, preferably at most 1:5, especially at most1:10, in relation to the extension thereof in two thereto and inrelation to each other perpendicular directions.

According to this invention it is possible to determine the friction ofa sheet-shaped test material, such as a sheet, against anothersheet-shaped material or the same kind of sheet-shaped material, butalso against a material with another shape than the sheet-shape, e.g. apiece of metal, plastics, elastomeric material, etc., having a suitableshape.

In the following the surface of the test material having sheet-shape isusually only called "test surface" and the surface of the materialagainst which the friction is measured is called "counter-surface" butalternatively the material of a friction surface forming body which doesnot exhibit "sheet-shape" can be the material for which the frictionagainst various other materials is to be determined.

2. Prior Art

A common way of determining the friction against a sheet-shaped materialis according to the level of the prior art to arrange the sheet-shapedmaterial on a flat surface acting as a support for the sheet-shapedmaterial, and measuring the force which is required for bringing acounter-surface forming body having a known weight to move on thesurface of the sheet-shaped material. The transverse or force which saidbody exerts against the surface (especially the normal force, i.e. aforce acting in a direction normal or perpendicular to said surface)depends on the weight of the body under the influence of the gravityforce. The sheet-shaped material can be arranged on a supportingsurface, the plane of which can be tilted in a degree which is requiredfor bringing the counter-surface forming body to start moving on saidsurface by the influence of the gravity force component acting in thedirection of the plane of said surface, whereby the slope of said planeagainst the horizontal plane when the body starts sliding gives ameasure of the force of friction and coefficient of friction inquestion, or else the body can be pulled on the preferably horizontallyarranged surface of the sheet-shaped material and the pulling force andnormal force and preferably also the surface load are measured when thebody starts to slide. Said previously known methods have, however,substantial disadvantages caused e.g. by uneven load of the force offriction, the appearance of jumping movement of the body, the creationof acceleration forces which disturb the measurement of the force offriction, that the application of the normal force may take place undera sidewise movement of the normal force and that the application of thenormal force may take place with jumps or with variations from oneexperiment to the other, and the measuring methods are oftentime-consuming and complicated, etc.

It has now surprisingly been revealed that it is possible to completelyor to an extential degree eliminate said deficiencies and also tosimplify the measurement with the process and the device according tothis invention.

OBJECT AND SUMMARY OF THE INVENTION

When performing friction measurements of the kind mentioned above theposition of the transverse, especially normal forces exerted on thesheet-shaped material is according to this invention maintainedessentially stationary. This permits the point (area) of action of saidforces to optionally be displaced in a direction which is perpendicularto the friction surface, e.g. in case a supporting substrate for saidsheet material is elastically resilient and compressible when subjectedto said normal forces. The area within which the normal force or forcesact is, on the contrary, preferably not at all or only to a slightdegree displaced in a direction which is perpendicular to the normalforces. This is a part from the movement caused by elastic deformationand apparatus play which may take place when applying a frictionmeasuring force on the friction surface of the sheet material, i.e. onthe surface area subjected to the normal force, in the direction of theplane of the friction surface. According to the invention it is alsopreferred that the test material can be moved only in a direction in itssurface plane by a force, especially a tensile force, which is exertedon the test material and acts in a direction in its surface plane whenmeasuring the friction. This can be achieved by having the test materialclamped in one or more clamping means (clamping jaws) during the testingprocedure. The clamping devices should be arranged so that the testedmaterial sample moves in the direction of the friction measuring forceonly, i.e. usually in the direction of a tensile force applied to thesample. This can be achieved by arranging the movable clamping meansmovable along a straight line or movement axis, the extension of saidline preferably lying in or passing very close to the plane of the testsurface or counter-surface and in parallel or very closely to parallelwith said surfaces. This can be achieved by clamping the test sampleand/or the counter-surface material piece e.g. in the shape of strip orstrips, rigidly in the clamping device or devices, especially themovable clamping device or devices, and especially against movements ormovement components which deviate from the straight line movements ofthe clamping devices, such as a rotating movement component in the planeof the friction surface/counter-surface. This can be achieved e.g. byusing a test sample or counter-surface sample respectively with asufficient breath, e.g. about 50 mm, e.g. for paper and materials,especially fibrous materials, which have a stiffness which is sufficientfor preventing the deviating movement mentioned above at least to anextent which is sufficient for a reliable friction measurement based ona straight line movement of the test surface and/or counter-surfacerespectively as mentioned above, especially along the extension of astraight line movement of a sample clamping device. Thus, in order toobtain said desired testing conditions the clamping devices 12, 15 onthe figure should normally be "stiff" and not flexible or tiltable, i.e.force the test sample surface and/or counter-surface to perform astraight line movement.

Furthermore, it is important that the shape of the device prevents thatthe tested material alternating sticks and performs jump movements. Tothis contributes also that the tested material can have a low mass whichcounteracts the influence of the mass forces on the measurement.Furthermore, the tested material can have a very small thickness whichmakes it possible to apply the tensile forces very close to the testedmaterial sheet so that the formation of bending momentum or forcecomponents perpendicular to the test surface acting on the tested sampleis counteracted.

For the friction measurement the sheet material is thus subjected toforces (friction measurement forces), especially tensile forces,especially such forces which are large enough to cause the appearance ordisappearance respectively of a movement or a beginning or continuous,even movement (slide) of the test material (sheet-shaped material) inrelation to the counter-surface material.

The size of said friction forces can be measured and registrated e.g.with a strain gauge (wire strain gauge). The transfer (the coupling) ofthe force from the means or device exerting or applying the frictionmeasuring force to the sheet material can be more or less elastic.Usually it is suitable or acceptable that said coupling (transfer) whenmeasuring the static friction is more elastic than when measuring thekinetic friction. The elasticity of the force transfer may be madevariable for an adaption to the type of friction. It is suitable to useclamping devices in which the sheet-shaped material is clamped, e.g.between clamping jaws, at least one of said clamping devices beingarranged displaceable in a direction which makes it possible to exert aforce on the sheet-shaped material in the direction of the area of thesheet within which the friction measurement is performed, especially atensile force.

The sheet-shaped material is preferably used in the testing process inthe shape of strips which are arranged between the opposite loadingsurfaces with at least one end of the strip extending outwards from saidsurfaces so that the strip can be clamped in the movable clamping meansfor exerting a force in the direction of the friction surface (thefriction measuring force). According to a suitable embodiment the loadsurfaces are arranged in the vertical direction or sloping in relationto the horizontal plane so that the sheet-shaped tested material,preferably in the shape of a strip, can easily be inserted into the gapbetween said surfaces under the influence of the gravity force, in whichcase preferably also the gap between the clamping means of the movableclamping device are directed parallel to the gap between the loadsurfaces, as is preferably also the gap between the clamping means inone or more optionally used further clamping devices used for clampingof a further sheet-shaped material (counter-surface material) used inthe testing process.

In the testing process also the counter-surface material may suitably besheet-shaped and used e.g. in the shape of a strip. The counter-surfacematerial may e.g. be the same kind of material as the tested material,e.g. paper. A strip of the sheet-shaped material having one end clampedin a movable, tensile force exerting clamping device, may in the gapbetween the load surfaces be arranged between two other pieces,preferably two strips or a double-folded strip of the sheet-shapedcounter-surface material, in which case said counter-surface material issuitably maintained stationary between the load surfaces, e.g. byholding an extending end of the counter-surface material or pieces in astationary (immovable) clamping device. This permits a simple way ofmeasuring the friction of e.g. paper against the same kind of paper. Thepaper web direction (machine direction) of the tested material and thecounter-surface material may then be directed in the same direction,directed 90° in relation to each other, 180° in relation to each other,or in other mutual directions, etc. and with various combinations of theupper sides and the lower sides (wire sides) of said sheets applyingagainst each other. When performing the measurement one may either applya normal force of a constant size against the friction surfaces of thesheet material and the counter-surface material and vary the frictionmeasuring force in the direction of the friction surface, or apply aconstant friction measuring force and vary the normal force until asliding movement appears or disappears respectively.

For measuring the friction against only one of the surfaces of thesheet-shaped material it is also possible to arrange one of the oppositeload surfaces movable in the direction of the friction surface plane,preferably with the lowest possible friction and preferably friction ofa known size, e.g. by arranging the movable load surface displaceable onroller bearings or similar devices.

It is usually also essential that the load surfaces which exert a normalforce against the sheet-shaped material are guided so that they aremaintained parallel to each other and preferably also to thesheet-shaped tested material and apply a load force (normal force)perpendicular to the sheet-shaped material, and especially to preventthat the load surfaces exert a sizing or gripping effect on thesheet-shaped material by the influence of a tensile force applied in thedirection of the sheet, i.e. that the tensile force applied formeasuring the friction creates a normal force component. The risks forthis can be decreased by applying the friction force in the plane of thefriction surface.

It is usually essential that the load surfaces are made from an elasticmaterial, especially a layer of elastic material on a preferably planesubstrate of a rigid material, such as a metal, so that an evenlydistributed contact pressure on the sheet-shaped material is obtained.

Furthermore, it is an advantage if the tested material, especially thetest strip, can move only in the direction of the applied tensile forceand that sliding in other directions is efficiently prevented, suitablyby rigid clamping in clamping devices in the measuring process. Theapparatus should, furthermore, as far as possible prevent that thesurfaces alternating are sized and alternating make jump-like movements,so-called stick-slip movement.

It is also important that the tested material and optionally thecounter-surface material are of small thickness so that the tensileforce (the friction force) can be applied in a direction close to or inthe plane of the friction surface, and also so that bending movements onthe tested material are avoided or decreased. A thickness of below 2 mm,usually below 1 mm or below 0.5 mm, e.g. below 0.2 mm is often a valuewhich gives good testing conditions, e.g. when testing paper (includingalso board, paperboard and similar).

According to the invention it is possible to use test pieces or sampleswith a very low weight which makes it possible to decrease the influenceof the mass forces on the friction measurement. When using astrip-shaped test sample material the strips may e.g. have a length ofat most 300 mm, e.g. at most 200 mm or at most 100 mm, and a minimumlength which is determined by the apparatus, usually at least 20 mm orat least 50 mm, in each case as single or double-folded material, andthe bredth may often be at most 100 mm or at most 75 mm, usually down toat least 5 mm or at least 10 mm or at least 25 mm, e.g. a bredth ofabout 50 mm, e.g. 150×50 mm or 300×50 mm, double-folded to half thelength, which is often suitable for paper.

A further advantage of the invention is that the tested material samplesmay be produced uncomplicated and efficiently, e.g. by cutting, punchingor stamping of strips of the sheet-shaped material, and that the testsamples can easily be introduced into the testing apparatus withouttouching the contact surfaces (friction surfaces) during the handling.

It is usually also suitable that the normal force, which acts betweenthe surfaces which are subjected to friction measuring, is applied undercontrolled conditions. Usually a gradual increase of the normal force isdesired, wherein the friction surfaces are brought together withoutmutual sliding movement. Furthermore, it is essential to control theperiod of time during which the normal force acts upon the frictionsurfaces before the measurement is performed.

When measuring the friction of such materials as paper and similar it isoften essential that the friction measurement is performed a repeatednumber of times on the same surfaces in order to obtaining a stable andreproducable result.

BRIEF DESCRIPTION OF THE DRAWINGS

On the enclosed drawing the Figures disclose:

FIGS. 1 and 2 an example of an apparatus which can be used for carryingout the process according to this invention;

on FIGS. 3 and 4 examples of the arrangement of double or simple stripsof sheet-shaped material, e.g. paper, which exhibit varyingcharacteristics in different directions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 discloses a normal force loading apparatus 1 with a fixed (duringmeasurement) normal force applying means 2 with a layer 3 of an elasticmaterial, which forms one of the load surfaces, and a movable and forceapplying normal force applying means 4 with a layer 5 of an elasticmaterial, which forms the other load surface. The load applying means 2which is fixed during the measurement may also be movable (displaceable)from the opposite force applying means 4 to a charging position in whichcharging (inserting) of the sheet-shaped material between the forceapplying means 2,4 is simplified, and back towards the opposite forceapplying means 4 to a fixed measurement position in which the fixedforce applying means 2 is loaded with (subjected to) the normal forceexerted by the movable force applying means 4. The normal force isaccording to the embodiment disclosed on the Figure, in which both loadsurfaces are vertically directed, obtained with a weight 6 which by alever 7, which is pivoted around a horizontal axis 8, transfers thegravity force redirected into the horizontal direction to the movableforce applying means 4, which is held by thin blade springs 9. A strip11 of a sheet-shaped material is arranged between the elastic layers 3and 5 respectively of the force applying means 2 and 4 respectively, andone end of the strip 11 is clamped into a movable tensile force applyingmeans 12 in a gap 13 between clamping means (clamping jaws). Thecounter-surface material consists of a double-folded strip 14 of asheet-shaped material, and the test material strip 11 is arrangedbetween the two legs thereof. The end of the strip 14 is clamped in afixedly arranged clamping device 15 in a gap 16 between clamping means.The test material may, however, alternatively consist of two singlestrips 11 laid tight together or of a double-folded strip and in bothembodiments be arranged between the two layers of a counter-surfacematerial 14. The tensile force acting as friction measuring force isexerted on the movable clamping device 12 by a tensile force device 17,outlined as a screw-nut device, with a driving means 19, shown as amotor which turns said screw. The tensile means device is through aforce measuring cell 18, e.g. a wire strain gauge, connected to theforce applying device 12. Signals from said measuring cell 18 are,through a calculating unit 20, transferred to a display-printer device21. The measured values of the normal force and friction measuring forceare used for calculating the coefficient of friction of the surfaces inquestion at the load condition in question.

FIGS. 3 and 4 disclose in outline an example of the mutual relationbetween test strips 11 and counter-surface strips 14 of sheet-shapedmaterial with characteristics which differ in various surface planedirections, typically paper, which was made in a paper-making machineand which discloses a "machine direction" and an "upper side" and a"lower side" which last-mentioned side faced a wire cloth or similar.The direction of the arrows shows the machine direction, and the arrowwith a double line shows the upper side and the arrow with a single linethe lower side, and full lines relate to surfaces turned towards theviewer and hatched lines surfaces turned from the viewer, respectivelyFIG. 3 corresponds to the test and counter-surface materials arrangementdisclosed on FIG. 1.

FIG. 4 discloses corresponding conditions when also the test materialexhibits double legs, in this case a double-folded strip.

As is immediately obvious also a number of other combinations ofdirections and sides can be achieved, wherein the machinery directionmay also be directed in an angle in relation to the longitudinaldirection of the strip, 90°.

The apparatus disclosed above has been used with good results formeasuring the friction of writing paper, copying paper, sack craftpaper, fibre board for corrugated fibre board, etc. against the samekind of material and against other kinds of material, such as aluminum,iron, polyethylene plastics, etc., and of metals, such as steel,aluminum, brass, etc., plastics, such as polyethylene, polyamides,polyesters, cloth, such as cotton, polyester, etc. against the samematerial and against other of the materials mentioned above, whereinmeasuring values with a low degree of spread was obtained.

I claim:
 1. A process for measuring friction of paper surfaces againstpaper surfaces for determining a coefficient of friction between saidsurfaces, which comprises arranging paper friction testing samples withcontacting plane friction test surfaces between two mutually parallelopposite plane load surfaces, exerting with the aid of said loadsurfaces, an evenly distributed contact force on the friction testsurfaces of the paper samples in a direction essentially normal to thefriction test surfaces, maintaining a first of said samples and the loadsurfaces stationary, exerting a friction measuring tensile force on asecond of said paper samples, the friction testing surfaces of which arein contact with the friction testing surfaces of said first sample, saidtensile force being exerted with movable paper sample clamping meansarranged for being moved in a direction of a straight line and forclamping said second paper sample for movement along the straight linewhich is in a direction of the friction test surfaces when measuring thefriction, and measuring the combination of said essentially normalforce, exerted with the aid of the load surfaces on the paper frictiontest surfaces, and said tensile force, which is required for a slidingmovement of the friction test surfaces of said clamped second paperfriction test sample relative to the contacting paper surfaces of saidfirst paper friction test sample for determining said coefficient offriction.
 2. A process according to claim 1 wherein the paper frictiontest samples consist of paper strips with a breadth of at least 10 mm.3. A process according to claim 1 wherein one of the essentially normalforce exerted on the paper friction test surfaces and the tensile forceacting on the second paper friction test sample is gradually changed sothat sliding (movement) or rest (non-movement) of the friction testsurfaces of the second friction test sample in relation to the firsttest sample is obtained.
 4. A friction measuring apparatus for carryingout the process according to claim 1 which apparatus comprises a loadingdevice with said two mutually opposite plane load surfaces, of which atleast one is movable in a direction which is transverse to said planeload surfaces for applying under controlled conditions, said essentiallynormal force perpendicular to said paper friction test surfaces arrangedbetween said load surfaces, said loading device being arranged with theload surfaces stationary, apart from movability in directions transverseto the load surfaces, said apparatus being provided with means forholding said first paper friction test sample stationary together withthe load surfaces, and at least one said clamping means, which ismovable in the direction of said straight line and arranged for holdingsaid second paper friction test sample clamped and for moving saidsecond paper friction test sample between said load surfaces in astraight line parallel to said plane load surfaces and parallel to thedirection of movement of the clamping means, by exerting said tensileforce on said second paper friction test sample, and means forcontrolling and measuring the essentially normal force exerted by theload surfaces against the paper friction test surfaces and the tensileforce exerted by the movable clamping means on the second paper frictiontest sample.
 5. An apparatus according to claim 4 wherein said samplesare strip-shaped paper friction test samples, and said clamping meanscomprise clamping jaws for holding the strip-shaped samples at one endthereof.